1. Field of the Invention
This invention relates to a process of making an aqueous emulsion of relatively high molecular weight polyolefin and to a hybrid polyolefin-polyurethane dispersion obtained therefrom.
2. Description of Related Art
The use of polyolefins such as polyethylene, polypropylene, copolymers of ethylene and alpha-olefins, and ethylene-propylene-diene terpolymer (EPDM) continues to grow at a rate faster than that of many other resins due to their low cost and excellent balance between design flexibility, mechanical properties and recyclability.
In order to improve the mechanical properties of the myriad of consumer goods made from the foregoing types of polyolefins, these resins are commonly reinforced with fillers, e.g., glass fibers. Glass fiber-reinforced polyolefins have better dimensional stability, tensile strength, flexural modulus, flexural strength, impact resistance and creep resistance than non-reinforced polyolefins. The glass fibers are generally treated with a sizing composition that performs several functions. It protects the fibers in a strand during processing and it has the capability to couple or adhere the glass fibers to the polyolefin. Aqueous polyolefin emulsions have been found to be useful for glass fiber treatment in polyolefin and thermoplastic polyolefin reinforcement. However, they are not good film-formers and require the use of another film-forming component to achieve additional useful properties such as good strand integrity.
Due to the generally poor photostability of polyolefins, parts made from, e.g., polypropylene and thermoplastic polyolefin, are not used for the exterior parts of automobiles without first being given a protective coating. Another reason for using coatings is the desire that parts made from these resins match the color and gloss of the main automobile body. However, most coating systems cannot be used on polypropylene and thermoplastic polyolefin substrates due to poor adhesion. This lack of adhesion can be attributed to the generally poor wettability, good solvent resistance and very low surface energy of the polyolefins polypropylene and thermoplastic polyolefin. Polypropylene and thermoplastic polyolefin belong to the group of polymers with the lowest surface energy, next to that of the polysiloxanes and fluoropolymers.
The most common approach to overcoming the technical difficulties encountered in the coating of polypropylene, thermoplastic polyolefin and similar polyolefins is to use an adhesion-promoting primer coating based on chlorinated polyolefin resin. Primers often contain significant amounts of volatile organic compounds (VOC). This is a significant disadvantage for a finisher who is required to comply with strict federal and state environmental emission standards. In addition, halogen-containing organics such as chlorinated polyolefin present environmental concerns of their own.
Therefore, it would be advantageous to replace a chlorinated polyolefin based primer with one having good adhesion to polypropylene and thermoplastic polyolefin rubber but lacking a significant VOC component and in addition, lacking a halogen-containing organic component.
It is known to use aqueous polyolefin emulsions for the treatment of fillers used in the reinforcement of polyolefins, as property-enhancing additives and as adhesion promoters.
U.S. Pat. No. 3,814,715 describes a glass fiber aqueous sizing composition containing a low molecular weight polypropylene or polypropylene-polyethylene mixture emulsion with, amongst other components, a coupling agent.
U.S. Pat. No. 4,283,322 describes a glass fiber sizing composition containing an emulsion of a blend of carboxylated isotactic and amorphous polypropylenes having weight average molecular weights of less than 10,000.
U.S. Pat. No. 4,507,430 describes an adhesion or coating material for a polyolefin resin which is derived from a hydrogenated polyalkadiene polyol and a polyisocyanate.
U.S. Pat. No. 4,728,573 describes glass fibers which have been chemically treated with a composition containing amino organo coupling agents, an aqueous emulsion of an acid or anhydride modified polyolefin having one or more surfactants, a stabilizer and a film forming polymer.
U.S. Pat. No. 5,004,784 describes a composition consisting of a blend of two incompatible thermoplastic polymers and an isocyanate reacting agent having a functionality of less than three.
U.S. Pat. No. 5,389,440 describes a sizing coating emulsion containing a high molecular weight crystalline polyolefin an acid material, a base, an emulsifying agent and water.
U.S. Pat. No. 6,008,286 describes a primer composition for application to polymeric substrates, the composition including an aliphatic isocyanate-containing compound, a nonhalogenated hydrocarbon polymer containing organic functional groups, a halogenated hydrocarbon polymer containing organic functional groups, e.g., a maleated chlorinated polyethylene, and an organic solvent.
U.S. Pat. No. 6,166,118 describes a process for emulsifying high molecular weight functionalized polyolefins using emulsifiers and an indirect pressure method. According to the process, a pre-emulsion concentrate with a solids content of 55–90% is produced in the first stage and subsequently diluted to a lower solids content.
U.S. Pat. No. 6,339,125 describes a polyurethane dispersion which is the reaction product of a prepolymer and water or hydrazine as a chain extender, the prepolymer being a reaction product of a hydroxylated polyalkadiene, a bis(hydroxyalkyl) quartenary ammonium-group-containing diol and a polyisocyanate.
U.S. Pat. No. 6,465,559 describes a sizing composition which includes an anionic or cationic polyurethane dispersion, a polyolefin wax and a coupling agent.
WO02/36654 describes an anionic polyurethane dispersion compatible with polyolefins which is a reaction product of a prepolymer and a chain extender.
Polyolefin emulsions known in the art do not provide for continuous film forming and adhesive properties. Therefore, there remains a need for a material in the form of an aqueous dispersion that would provide both film formation and good adhesion to polyolefin substrates.